This invention relates to a process for obtaining low reacted color (interchangeably referred to herein as "low color") crude trimethylolpropane (TMP).
Trimethylolpropane is produced by the condensation and cross Cannizzaro reactions of n-butyraldehyde and formaldehyde in the presence of a strong alkaline hydroxide, such as caustic solution. Removal of the TMP from the reactor solution can be carried out by the partition of the product between the aqueous solution and an organic solvent for the TMP, e.g., ethyl acetate, isobutanol, butyl acetate, and the like. The TMP is then removed from the organic layer, as for example by distillation. Another method for the removal of the TMP from the organic layer and the further removal of caustic from the TMP is to add a second solvent, one in which the TMP is insoluble, in an amount sufficient to separate the aqueous but not sufficient to separate out the TMP, remove the water layer, and then remove the first solvent by distillation. Typically the second solvent is a non-polar solvent, e.g., xylene. The TMP/second organic solvent mixture is allowed to settle and the separated TMP is removed by decantation. See Palmer et al., U.S. Pat. No. 3,956,406. These processes generally produce a product which has an acid-wash color of about 5-10 Gardner Units (GU) or phthalic anhydride color of about 100-300 APHA. Historically, the art has obtained low reacted color TMP by extracting, leaching, or further purifying color-causing impurities generated in the reaction to produce TMP.
Cheung in U.S. Pat. No. 5,603,835 discloses a process which extracts color-causing impurities generated in the reaction to produce TMP, the process comprising extracting purified final TMP with an organic solvent wherein the color bodies are soluble in, but not the TMP. This extraction is reported to result in greater than about 85% yield of TMP product having an acid-wash color of 3 or less Gardner Units. The phthalic anhydride color of the TMP product is also reported to be less than about 100 APHA.
Low color TMP is not analogous to high purity TMP. The acid wash color of TMP is determined by extracting TMP with an organic solvent, followed by washing the extract with sulfuric acid, and monitoring the reacted color, e.g., through a colorimeter, to obtain a Gardner Unit (GU) value. The purity of a TMP sample is generally increased/enhanced by (re)crystallization techniques, or the like. Recrystallization may remove salts generated by the reaction of formaldehyde and butyraldehyde, but not necessarily remove color body impurities found in the TMP product. Palmer (US '406) addressed enhanced sample purity of TMP, while Cheung (US '835) addressed enhanced color of TMP. Both US '406 and US '835 involve a type of extraction process for the TMP product desired.
Other processes have produced product having an acid-wash color of about 5-6 Gardner Units. However, for many applications it is desirable to obtain TMP product having as low an acid wash color, or other color analyses, as possible and this low color obtained in an economically efficient manner. Thus the art is continuously searching for methods to obtain low color TMP.